1. Field of the Invention
The present invention relates to a semiconductor device including a circuit constituted by thin film transistors (hereinafter referred to as TFTs) and a method of fabricating the same. For example, the invention relates to an electro-optical device typified by a liquid crystal display panel, and an electronic apparatus incorporating such an electro-optical device as a component.
Note that in the present specification, the term “semiconductor device” indicates any devices capable of functioning by using semiconductor characteristics, and all of the electro-optical devices, semiconductor circuits, and electronic apparatuses are semiconductor devices.
2. Description of the Related Art
In recent years, attention has been paid to a technique to construct a thin film transistor (TFT) by using a semiconductor thin film (thickness of several to several hundreds nm) formed over a substrate having an insulating surface. The thin film transistor is widely used for an electronic device such as an IC or electro-optical device, and its development is hastened especially as a switching element of an image display device.
As a semiconductor thin film forming an active layer of a TFT, although a noncrystalline silicon film (typically an amorphous silicon film) has been often used, a demand for a TFT having a faster operation speed is increased, and a crystalline silicon film (typically a polysilicon film) has become the mainstream. As a technique for obtaining the crystalline silicon film, a method in which after an amorphous silicon film is formed, the film is crystallized by a heat treatment or irradiation of laser light, is often used.
Besides, there is disclosed a technique (Japanese Patent Unexamined Publication No. Hei. 6-232059 and No. Hei. 7-321339) in which after an amorphous silicon film is formed, a catalytic element (for example, nickel) for promoting crystallization of the amorphous silicon film is introduced, and a heat treatment is carried out to obtain a crystalline silicon film. According to this technique, it is possible to obtain a uniform crystalline silicon film in a short time.
However, the catalytic element for promoting crystallization of the amorphous silicon film often deteriorates the characteristics of the TFT. Then, after crystallization, a region where the catalytic element exists at a high concentration is removed by etching or the like.
Hereinafter, a specific description will be made on a crystallizing technique using a catalytic element for promoting crystallization of an amorphous silicon film, and a technique for removing a region where the catalytic element exists at a high concentration.
In FIGS. 1A and 1B, reference numeral 101 designates a silicon film; 102, a beltlike region on a silicon film surface (hereinafter referred to as a catalytic element introduction region); and 103, a silicon oxide mask covering the silicon film surface other than the catalytic element introduction region. Note that by using the silicon oxide mask 103, the catalytic element is selectively introduced into the catalytic element introduction region 102.
First, the catalytic element is introduced into the catalytic element introduction region 102, and by carrying out a heat treatment, crystals are made to grow from the catalytic element introduction region 102 in a direction parallel to an insulating surface and a direction almost vertical to a long side of the catalytic element introduction region 102. Note that reference numeral 104 designates the direction of crystal growth.
A leading end portion of crystal growth obtained in this way is designated by 105. It is known that the catalytic element of high concentration exists in the leading end portion 105 of the crystal growth. When a crystal growth distance exceeds some value, a region where an active layer of a TFT can be disposed is formed between the beltlike catalytic element introduction region 102 and the leading end portion 105 of the crystal growth where the catalytic element exists at a high concentration.
Next, when the active layer of the TFT is formed using the region sandwiched between the leading end portion 105 of the crystal growth and the beltlike catalytic element introduction region 102, other regions (including at least the leading end portion 105 of crystal growth) where the catalytic element exists at a high concentration are removed by etching.
Conventionally, the arrangement of the catalytic element introduction region is determined so that a region which becomes an active layer of a TFT in a subsequent step exists in the region sandwiched between the leading end portion 105 of the crystal growth and the beltlike catalytic element introduction region 102, and a heat treatment condition for crystallization is determined.
Conventionally, it has been considered to be appropriate that the arrangement of the catalytic element introduction region is determined so that the region which becomes the active layer of the TFT in the subsequent step exists in the region sandwiched between the leading end portion of the crystal growth and the catalytic element introduction region. Besides, even if the catalytic element is removed in a step subsequent to crystallization, since it is difficult to completely remove the catalytic element, it has been considered to be sufficient if a necessary minimum amount of catalytic element is introduced.
Thus, one catalytic element introduction region has been provided at one side of the region which becomes the active layer of the TFT in the subsequent step. Note that a crystal growth velocity at 570° C. in the case where only one catalytic element introduction region (width w=10 μm) was disposed was about 3 μm/hr.